WO 88/01,938 has disclosed a process of the generic type for producing base materials copper-clad on one or both sides by the so-called filament-winding technique and the use thereof for producing multilayers. In this process, the winding mandrel used is a steel sheet covered with a copper foil. This sheet is wound at the smallest possible inclination with one ply of yarn filaments of a reinforcing material, preferably alkali-free glass, initially across the width, with the winding mandrel rotating. After the whole surface of the sheet has been wound, this winding step is repeated at an angle of 90.degree., whereby a second ply is applied. The step of crosswise winding is repeated several times until the desired glass content or the desired thickness of the wound plies has been reached. Instead of the conventional fabric reinforcement, a laid fiber structure is thus formed, in which the individual filaments are not woven with one another but lie loosely on top of one another.
After completion of the winding step, a closed mold is formed by means of two metal sheets, covered on the inside with copper foil, and lateral sealing webs, which mold is evacuated to a residual pressure of less than 2 mm Hg. In an injection process or casting process, which is known to those skilled in the art, for example, as the RTM process (resin transfer molding), a prepared and, if appropriate, heated resin mixture is then allowed to flow in and, after the filling step has been completed, the resin mixture is allowed to cure fully at elevated temperature.
The casting resin mixture, described in WO 88/01,938, of epoxide resin and anhydride hardening agent has, however, a poor adhesion between copper and the plies, since a hardening agent must be added in relatively large quantities, and this impairs the adhesive force (peel strength).
In particular in the case of base material with a thin copper foil, for example 17.5 /um thickness, whose use is indispensable for fine line technology and very fine line technology, the peel strength drops down to 1.0N/mm and lower, so that the requisite values are not reached.